Many integrated circuit (IC) devices output and receive information in digital form--i.e., bits of data representing logical "1"s and logical "0"s. For output information, the voltage level for a logical "0" is generally referred to as "voltage output low" or "VOL," while the voltage level for a logical "1" is generally referred to as "voltage output high" or "VOH." For input information, the voltage level for a logical "0" is generally referred to as "voltage input low" or "VIL," while the voltage level for a logical "1" is generally referred to as "voltage input high" or "VIH."
Traditionally, the voltage level of VOH for most IC devices fell within a pre-defined range, thus ensuring that the data output of one IC device could be correctly interpreted by another IC device. Due to power considerations, however, many IC devices are now being implemented so that the maximum voltage level of VIH is lower than the maximum VOH value of the traditional, pre-defined range.
As a practical matter, this means that a system which incorporates a number of different IC devices may not function properly unless the differences in respective voltage levels for VOH have been resolved.